The question does most of the answering: "What Is The Most Important Invention In The Past Two Thousand Years?"

That lets out agriculture, writing, mathematics, and money. Too early.

"Most important" would suggest looking for inventions near the beginning of the period, since they would have had the most time for accumulative impact.

Where did that number "Two Thousand" come from? From the approaching Year 2000, which is a Christian Era date — now referred to as "Common Era": 2000 CE. That's quite a clue.

The most important cultural — hence all-embracing — invention is a religion. Only two major religions have been invented in the last two millennia, Christianity and Islam. Try to imagine the last two millennia, or the present, without them.